Bringing works of art to light

The history of Europe is not only recorded in history books but also in its great works of art. People from all over the world travel to various cities across Europe to gaze in wonderment at these works of art, created by the likes of Hogarth, Brandl or El Greco. While experts try their utmost to protect them, time has not been so kind, resulting in the need for restoration. During this process, lasers are often used to passively peer behind the dust and dirt that has accumulated over the ages. Digital scans have also been used and scientists have even managed to see the eyebrows of Mona Lisa, painted by Leonardo da Vinci. Now a team of Italian researchers has developed a novel imaging tool that can capture features not otherwise detectable with the naked eye or current imaging techniques, refining the restoration process even further.

'This is, to the best of our knowledge, the first time that this technique has been applied on artworks,' said Dario Ambrosini of the University of L'Aquila in Italy, one of the paper's authors. 'This novel method represents a powerful yet safe tool for artwork diagnostics.' The technique relies on the fact that all objects emit some infrared radiation. Depending on their temperature, certain materials shine more brightly in one wavelength than in others. At normal room temperature, paintings typically emit more energy in the longer infrared wavelengths (42%) than they do in the mid-infrared (1.1%).

The laser system is known as thermal quasi-reflectography (TQR) and is able to create revealing images using reflected light from the mid-infrared part of the spectrum (three to five micrometres in wavelength). This allows them to study intricate details, analyse pigments, and search for subtle defects not visible to the naked eye.

Researchers from the University of L'Aquila, the University of Verona, and Italy's National Institute of Optics in Florence successfully demonstrated the TQR system on two famous works of art: the Zavattari frescos in the Chapel of Theodelinda and "The Resurrection" by the Italian Renaissance artist, Piero della Francesca. The researchers detailed their work in a paper recently published in the Optical Society's (OSA) open-access journal Optics Express.

Traditional infrared imaging, such as thermography, detects subtle temperature differences due to the pigmentation on the surface of paintings. These thermal maps can be used during art restoration to reveal internal defects that are not evident in visible light.

The TQR imaging system, however, uses a very different tactic and does not detect heat emitted from paintings at all; in fact it tries to minimise it. The TQR system operates a bit like radar, only with light. It shines a faint mid-infrared light source onto the surface of the painting and records the light that is reflected back to a camera.

In its first test on a small section of the Zavattari frescos in the Chapel of Theodelinda, the TQR system revealed details that were missed by earlier optical and near-infrared studies. 'Our system easily identified old restorations in which missed gold decorations were simply repainted,' said lead author Claudia Daffara of the University of Verona. 'The TQR system was also much better at visualising armour on some of the subjects in the fresco.'

To further evaluate the potential of the TQR system the researchers also studied a painting known as "The Resurrection" by Piero della Francesca. The TQR system identified interesting features, such as highly reflective retouches from previous restorations, all while operating during normal museum hours without interruption. The most surprising feature was an area around a soldier's sword that was painted by using two different fresco techniques. This subtle distinction was not detected by NIR photography.

'For mural paintings the use of the mid-infrared regions reveals crucial details,' said Dr Daffara. 'This makes TQR a promising tool for the investigation of these artworks.'

The researchers are currently conducting tests to determine if the TQR system can also provide infrared spectra of the surface of paintings, which may be able to identify the pigments used. 'Determining the chemical makeup of the pigments is important in determining how best to protect and restore the artwork,' said Dr Ambrosini.

According to the researchers, TQR may have applications beyond art preservation. 'In principle, it should work whenever we desire to differentiate surface materials,' said Ambrosini.